A liquid crystal display device has high image quality, thin size, light weight, low power consumption, and other features and therefore is being widely used for televisions, personal computers, car navigation systems, etc. A liquid crystal display device has two polarizers arranged above and below liquid crystal cells so that their transmission axes perpendicularly intersect and applies voltage to the liquid crystal cells so as to change the orientations of the liquid crystal molecules and display an image on a screen. In a twisted nematic mode liquid crystal display device, at the time of application of voltage, the liquid crystal molecules are oriented vertically and black is displayed in most configurations. In an in-place switching (IPS) mode liquid crystal display device, at the time of no application of voltage, the liquid crystal molecules are oriented in a certain direction, while at the time of application of voltage, the direction of orientation rotates 45 degrees and white is displayed in most configurations.
In a liquid crystal display device where two polarizers are arranged so that their transmission axes perpendicularly intersect in the longitudinal and lateral directions, when viewing the screen from the longitudinal and lateral directions, sufficient contrast is obtained. However, when viewing the screen from a direction off from the longitudinal and lateral directions, the transmission axis of the inlet side polarizer and the transmission axis of the outlet side polarizer no longer perpendicularly intersect in appearance, so linear polarized light cannot be completely blocked, light leakage occurs, sufficient black is not obtained, and the contrast ends up falling. For this reason, attempts are being made to add an optical compensating means to the liquid crystal display device so as to prevent a drop in the contrast of the screen.
Such optical compensating means have been developed and improved on in the past. For example, Japanese Patent Publication (A) No. 2004-133313 discloses an optical laminate comprised of an “A” layer containing a material with a negative intrinsic birefringence as a main ingredient on at least one surface of which a “B” layer containing a transparent resin as a main ingredient is laminated, in which optical laminate a variation of in-plane direction retardation Re is within ±10 nm. Further, Japanese Patent Publication (A) No. 2005-274725 discloses an optical laminate comprised of an “A” layer containing a resin with a negative intrinsic birefringence value on at least one surface of which a substantially non-oriented “B” layer comprised of a transparent resin is laminated, in which optical laminate the in-plane direction retardation Re of the “A” layer is set to a value over the in-plane direction retardation Re of the “B” layer. Furthermore, Japanese Patent Publication (A) No. 2007-72201 discloses an anisotropic stretched film containing a styrene-based resin in 50 wt % or more, which has a heat shrinkage stress at a predetermined temperature, in-plane direction retardation, and orientation angle set to predetermined ranges of variation.
The styrene-based resins used in these Patent Literatures have high transparency and negative intrinsic birefringence values, so are considered effective materials for expressing desired optical characteristics. However, there is the problem that films using these are extremely fragile and often break or wrinkle during the continuous transport at the time of stretching for expressing the desired optical characteristics, are no good in workability (stretchability), and are low in the durability of the stretched film after working. Further, there is the problem that reduction of the variation in in-plane direction retardation Re or the variation in orientation angle θ is difficult and that it is difficult to produce a stretched film superior in optical uniformity.
To deal with these problems, the above-mentioned Japanese Patent Publication (A) No. 2007-72201 sets the difference of the heat shrinkage stress at the (Vicat temperature+30° C.) in the flow (MD) direction and the width (TD) direction at 0.5 to 8.0 MPa, but the results are still not sufficient. Further, in the above-mentioned Japanese Patent Publication (A) No. 2004-133313 or the above-mentioned Japanese Patent Publication (A) No. 2005-274725, the improvement is made by laminating on at least one surface of a layer comprised of a styrene-based resin a transparent resin layer and stretching the laminate. These can be said to be effective techniques, but there is room for further improvement.
Further, to eliminate the dependency on the angle of the field of vision, a stretched film for producing a particularly suitable optical device used for an IPS type liquid crystal display device changing the direction of the electric field from the conventional longitudinal one to a lateral one and switching the liquid crystal molecules while leaving them parallel to the substrate surface has been sought.
The present invention was made in consideration of this point and has as its object the provision of a stretched film superior in stretchability, durability, and optical uniformity.
Further, it has as its object the provision of a stretched film for producing a suitable optical device used for an IPS type liquid crystal display device and an IPS type liquid crystal display device excellent in display properties.